System and method for increasing the strength of a bond made by a small diameter wire in ball bonding

ABSTRACT

A system and method is disclosed for increasing the strength of a bond made by a small diameter wire in ball bonding. In one embodiment of the invention a structure for receiving a ball bond comprises substrate material that has portions that form a substrate cavity and a wire bond pad that covers and fills the substrate cavity. The wire bond pad also has portions that form a wire bond cavity for receiving the ball bond. The ball is wirebonded to the sides and bottom of the wire bond cavity. The sides of the wire bond cavity provide additional strength to the bond to resist shear and tensile forces that may act on the wire.

This application is a Division of U.S. application Ser. No. 10/611,578filed on Jul. 01, 2003 and issued as U.S. Pat. No. 6,908,787.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to integrated circuitsand, more specifically, to a system and method for increasing thestrength of a bond made by a small diameter wire in a ball bondingprocess.

BACKGROUND OF THE INVENTION

Wirebonding is a well known electrical interconnection technique used inthe manufacture of integrated circuits and other microelectronicstructures. For example, the input/output (I/O) wires of an integratedcircuit chip may be wirebonded to their respective wire bond pads on theintegrated circuit chip.

Wirebonding is a solid phase welding process that welds the metallicmaterial of a wire to the metallic material of a corresponding wire bondpad. After a wire and a wire bond pad are placed in contact, the wireand the wire bond pad may be welded together using a well known processsuch as a thermocompression process, an ultrasonic process, or athermosonic process.

There are two basic forms of wirebond. They are the ball bond form andthe wedge bond form. Ball bonding and wedge bonding are both well knownin the wirebonding art. The most commonly used elements used to makebonding wire are gold, aluminum, and copper. Gold, aluminum, and copperare strong, ductile, flexible, reliable, and have similar values ofelectrical resistance. Gold wire bonding is the most widely usedwirebonding technology in the integrated circuit industry.

FIG.1 illustrates a cross sectional view 100 of a prior art wire bondpad 120 and a prior art ball bond 150 of wire 140 bonded to the wirebond pad 120. Wire bond pad 120 is made of a metal conductor such asaluminum or copper. During the process of fabrication, wire bond pad 120is placed on substrate material 110 (e.g., silicon). The edges of wirebond pad 120 are then covered with passivation material 130. A centralportion of the external surface of wire bond pad 120 is open to receiveball 150 on the end of wire 140. Ball 150 is placed on wire bond pad 120and bonded to wire bond pad 120 using a prior art wirebonding technique.

Integrated circuit designers continue to increase the number offunctions that integrated circuits are capable of performing. Theincreased number of integrated circuit functions has also increased thenumber of input/output (I/O) wires that must be connected to anintegrated circuit. An increase in the number of input/output (I/O)wires for an integrated circuit die also increases the area of anintegrated circuit die that must be allocated to the attachment sitesfor the input/output (I/O) wires.

It is desirable to minimize the amount of area of the integrated circuitdie that must be allocated to the attachment sites for the input/output(I/O) wires. One possible approach is to (1) reduce the size of the wirebond pads on the integrated circuit die, and (2) reduce the size of thediameters of the input/output (I/O) wires, and (3) reduce the size ofthe ball bond on the end of each input/output (I/O) wire.

The use of input/output (I/O) wire having smaller diameters and the useof a smaller ball bond means that there will be a smaller contact areabetween the ball bond and the wire bond pad. A smaller contact areabetween the ball bond and the wire bond pad means that the bond strength(i.e., the shear and tensile strength) between the ball bond and thewire bond pad will be lower than the bond strength of a larger ball bondand a larger wire bond pad. The lower bond strength is an undesirablefeature.

There is therefore a need in the art for an improved system and methodfor increasing the bond strength of a bond between a ball bond of asmall diameter wire and a wire bond pad in a ball bonding process for anintegrated circuit chip. There is also a need in the art for anintegrated circuit chip that has a plurality of wire bond pads that arecapable of providing increased bond strength to a ball bond of a smalldiameter wire.

SUMMARY OF THE INVENTION

To address the deficiencies of the prior art, it is a primary object ofthe present invention to provide an improved system and method forincreasing the strength of a bond made to small diameter wires in a ballbonding process.

One advantageous embodiment of the present invention comprises astructure for receiving a ball bond. The structure comprises substratematerial that has portions that form a substrate cavity. The structurealso comprises a wire bond pad metal that covers and fills the substratecavity. The wire bond pad also has portions that form a wire bond cavityfor receiving the ball bond. When a ball on the end of a wire iswirebonded to the sides and bottom of the wire bond cavity, the sides ofthe wire bond cavity provide additional strength to the bond to resistshear and tensile forces that may act on the wire.

It is an object of the present invention to provide an improved systemand method for providing a structure for receiving a wire bond.

It is also an object of the present invention to provide an improvedsystem and method for increasing the strength of a bond made to smalldiameter wires in a ball bonding process.

It is another object of the present invention to provide an improvedsystem and method for providing a structure for receiving a wire bond inwhich the structure resists shear and tensile forces that may act on thebond, the interface between the wire and the wire bond pad.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention so that those skilled in the art maybetter understand the detailed description of the invention thatfollows. Additional features and advantages of the invention will bedescribed hereinafter that form the subject of the claims of theinvention. Those skilled in the art will appreciate that they mayreadily use the conception and the specific embodiment disclosed as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. Those skilled in the art willalso realize that such equivalent constructions do not depart from thespirit and scope of the invention in its broadest form.

Before undertaking the Detailed Description of the Invention below, itmay be advantageous to set forth definitions of certain words or phrasesused throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, whether such a device is implemented inhardware, firmware, software or some combination of at least two of thesame. It should be noted that the functionality associated with anyparticular controller may be centralized or distributed, whether locallyor remotely. Definitions for certain words and phrases are providedthroughout this patent document, and those of ordinary skill in the artwill understand that such definitions apply in many, if not most,instances to prior uses, as well as to future uses, of such definedwords and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, wherein likenumbers designate like objects, and in which:

FIG. 1 illustrates a cross sectional view of a prior art wire bond padand a prior art ball bond of a wire bonded to the prior art wire bondpad;

FIG. 2 illustrates a cross sectional view of an advantageous embodimentof a first type of wire bond pad in accordance with the principles ofthe present invention;

FIG. 3 illustrates a cross sectional view of a wire bonded to the firsttype of wire bond pad of the present invention shown in FIG. 2;

FIG. 4 illustrates a cross sectional view of an advantageous embodimentof a second type of wire bond pad in accordance with the principles ofthe present invention;

FIG. 5 illustrates a cross sectional view of a wire bonded to the secondtype of wire bond pad of the present invention shown in FIG. 4;

FIG. 6 illustrates a flow chart of an advantageous embodiment of amethod of the present invention for making a first type of wire bond padfor use in an integrated circuit;

FIG. 7 illustrates a flow chart of an advantageous embodiment of amethod of the present invention for making a second type of wire bondpad for use in an integrated circuit; and

FIG. 8 illustrates a sectional side view of an exemplary integratedcircuit package manufactured in accordance with the principles of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 8, discussed below, and the various embodiments used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the present invention may beimplemented for any suitably arranged integrated circuit.

The system and method of the present invention provides increased bondstrength between a ball bond of a small diameter wire and a wire bondpad in a ball bonding process for an integrated circuit chip.

FIG. 1 illustrates a cross sectional view 100 of a prior art wire bondpad 120 and a prior art ball bond 150 of wire 140 bonded to the priorart wire bond pad 120. The features of prior art wire bond pad 120 havebeen previously discussed. Prior art wire bond pad 120 is flat. Priorart wire bond pad is placed on the surface of substrate material 110.

FIG. 2 illustrates a cross sectional view of an advantageous embodiment200 of a wire bond pad 230 in accordance with the principles of thepresent invention. Substrate material 205 is formed having a substratecavity 210 formed within the surface 215 of substrate material 205. Inone advantageous embodiment the shape of substrate cavity 210 is acircle having a diameter 220. Other shapes for substrate cavity 210 mayalso be used.

During the process of fabrication, wire bond pad 230 is placed on thesurface 215 of substrate material 205 over substrate cavity 210. Theportions of wire bond pad 230 placed over substrate cavity 210 fill insubstrate cavity 210. As shown in FIG. 2, the portions of wire bond pad230 that fill in substrate cavity 210 form a wire bond cavity 240 in thesurface of wire bond pad 230. In one advantageous embodiment the shapeof wire bond cavity 240 is a cylinder having diameter 250 and depth 260.Other shapes for wire bond cavity 240 may also be used.

The edges of wire bond pad 230 are then covered with passivationmaterial 270. A central portion of the external surface of wire bond pad230 is open to receive a ball bond on the end of a wire.

FIG. 3 illustrates a cross sectional view 300 of a wire 310 bonded towire bond pad 230. Ball 320 is placed on wire bond pad 230 andwirebonded to wire bond pad 230 using a prior art wirebonding technique.Portions of ball 320 fill wire bond cavity 240 and are bonded to thesides and bottom of wire bond cavity 240. This increases the bond area(and increases the bond strength) between ball 320 and wire bond pad230. The sides of wire bond cavity 240 provide additional strength toresist shear forces that may act on wire 310. The increase in bond areaalso increases the tensile strength of the bond in direct proportion.

In addition, the portions of wire bond pad 230 that are located withinsubstrate cavity 210 of substrate material 205 (i.e., below the surface215 of substrate material 205) also increase the overall bond strengthbetween wire 310 and substrate material 205. The portions of wire bondpad 230 that fill substrate cavity 210 are bonded to the sides andbottom of substrate cavity 210. This increases the bond area (andincreases the bond strength) between wire bond pad 230 and substratematerial 205. The sides of substrate cavity 210 provide additionalstrength to resist shear forces that may act on wire 310.

Wire bond pad 230 of the present invention provides a bond strength forwire 310 that is greater than the bond strength that prior art wire bondpad 120 is able to provide to wire 140. This enables smaller diameterwires (and smaller diameter wire bond pads) to be used.

A second advantageous embodiment of the present invention is shown inFIGS. 4 and 5. The second embodiment uses a deeper substrate cavitywithin the substrate material than that used by the first embodiment ofthe invention. For this reason wire bond pad 230 may be referred to as a“shallow” wire bond pad because the depths of substrate cavity 210 andwire bond cavity 240 are relatively shallow compared to the deeperdepths of the cavities of the second embodiment.

FIG. 4 illustrates a cross sectional view of a second advantageousembodiment 400 of a wire bond pad 430 in accordance with the principlesof the present invention. Substrate material 405 is formed having asubstrate cavity 410 formed within the surface 415 of substrate material405. In one advantageous embodiment the shape of substrate cavity 410 iscylindrically symmetrical with the bottom of substrate cavity 410 havinga diameter 420. Other shapes for substrate cavity 410 may also be used.

As shown in FIG. 4, the diameter of the bottom of substrate cavity 410may be formed to be greater than the diameter of the top of substratecavity 410. This forms a restraining edge 425 around the circumferenceof substrate cavity 410. Restraining edge 425 provides additional bondstrength to any wire that is bonded to wire bond pad 430.

During the process of fabrication, wire bond pad 430 is placed on thesurface 415 of substrate material 405 over substrate cavity 410. Theportions of wire bond pad 430 placed over substrate cavity 410 fill inthe bottom portions of substrate cavity 410. As shown in FIG. 4, theportions of wire bond pad 430 that fill in the bottom portions ofsubstrate cavity 410 form a wire bond cavity 440 in the surface of wirebond pad 430.

In one advantageous embodiment the shape of wire bond cavity 440 iscylindrically symmetrical having a depth 460 and a bottom diameter 450.Other shapes for wire bond cavity 440 may also be used. The diameter 450of the bottom of wire bond cavity 440 is greater than the diameter ofthe top of wire bond cavity 440. The portions of wire bond pad 430 thatcover restraining edge 425 form a restraining edge, 455 around thecircumference of wire bond cavity 440. Restraining edge 455 providesadditional bond strength to any wire that is bonded to wire bond pad430.

The edges of wire bond pad 430 are then covered with passivationmaterial 470. Wire bond cavity 440 in the central portion of wire bondpad 430 is open to receive a ball bond on the end of a wire.

FIG. 5 illustrates a cross sectional view 500 of a wire 510 bonded towire bond pad 430. Ball 520 is placed on wire bond pad 430 andwirebonded to wire bond pad 430 using a prior art wirebonding technique.Portions of ball 520 fill wire bond cavity 440 and are bonded to thesides and bottom of wire bond cavity 440. This increases the bond area(and increases the bond strength) between ball 520 and wire bond pad430. The sides of wire bond cavity 440 provide additional strength toresist shear forces that may act on wire 510. The sides of wire bondcavity 440 also provide additional tensile strength to the bond.

In addition, the portions of wire bond pad 430 that are located withinsubstrate cavity 410 of substrate material 405 (i.e., below the surface415 of substrate material 405) also increase the overall bond strengthbetween wire 510 and substrate material 405. The portions of wire bondpad 430 that fill substrate cavity 410 are bonded to the sides andbottom of substrate cavity 410. This increases the bond area (andincreases the bond strength) between wire bond pad 430 and substratematerial 405. The sides of substrate cavity 410 provide additionalstrength to resist shear forces that may act on wire 510.

The portions of ball 520 that fill wire bond cavity 440 underrestraining edge 455 of wire bond cavity 440 form a restraining wedge530. Restraining wedge 530 engages restraining edge 455 of wire bond pad430 to provide additional bond strength to the bond between wire 510 andwire bond pad 430.

Wire bond pad 430 of the present invention provides a bond strength forwire 510 that is greater than the bond strength that prior art wire bondpad 120 is able to provide to wire 140. This enables smaller diameterwires (and smaller diameter wire bond pads) to be used.

Wire bond pad 430 forms a wire bond cavity 440 that is deeper than wirebond cavity 240 of wire bond pad 230. For this reason wire bond pad 430may be referred to as a “deep” wire bond pad and wire bond pad 230 maybe referred to as a “shallow” wire bond pad. It is understood thatcavities of varying depths may be used in practicing the presentinvention. The shallow wire bond pad 230 and the deep wire bond pad 430are examples of advantageous embodiments of the present invention.

FIG. 6 illustrates a flow chart of an advantageous embodiment of amethod of the present invention for making and using a first type ofwire bond pad for an integrated circuit. The steps of the method aregenerally denoted with reference numeral 600.

The first step is to fabricate substrate material 405 and form substratecavity 410 in an external surface 415 of substrate material 405 (step710). Substrate cavity 410 is then filled with portions of wire bond pad430 to form wire bond cavity 440 in wire bond pad 430 (step 720).

Then the edge portions of wire bond pad 430 are covered with passivationmaterial 470 (step 730). Then ball 520 of wire 510 is wirebonded to thesides and bottom of cavity 440 (step 740). A retaining wedge 530 isformed with the portions of ball 520 that fill cavity 440 underrestraining edge 455 (step 750).

FIG. 7 illustrates a flow chart of an advantageous embodiment of amethod of the present invention for making and using a second type ofwire bond pad for an integrated circuit. The steps of the method aregenerally denoted with reference numeral 700.

The first step is to fabricate substrate material 405 and form substratecavity 410 in an external surface 415 of substrate material 405 (step710). Substrate cavity 410 is then filled with portions of wire bond pad430 to form wire bond cavity 440 in wire bond pad 430 (step 720).

Then the edge portions of wire bond pad 430 are covered with passivationmaterial 470 (step 730). Then ball 520 of wire 510 is wirebonded to thesides and bottom of cavity 440 (step 740). A retaining wedge 530 isformed with the portions of ball 520 that fill cavity 440 underrestraining edge 455 (step 750).

FIG. 8 illustrates how the wire bond pads of the present invention maybe used in an integrated circuit package. FIG. 8 illustrates a sectionalside view of an exemplary integrated circuit package 800 manufactured inaccordance with the principles of the present invention. Integratedcircuit package 800 comprises substrate 820, integrated circuit die 830,and protective material 840. Substrate 820 comprises core layer 850,metal layer 860, and dielectric layer 870. As shown in FIG. 8,electrical leads 880 connect integrated circuit die 830 with metal layer860. The ends of the electrical leads 880 are bonded to a plurality ofwire bond pads 890 of the present invention.

Consider a prior art ball bonding process in which the diameter of awire bond pad opening is one hundred microns (100 μm). A micron is onemillionth of a meter (10⁻⁶ m). Therefore, one hundred microns (100 μm)is one ten thousandth (10⁻⁴ m) of a meter. The diameter of a typicalprior art wire that is bonded to a one hundred micron (100 μm) wire bondpad opening ranges from twenty five microns (25 μm) to thirty twomicrons (32 μm).

If the diameter of the wire bond pad opening is reduced to fifty microns(50 μm), the diameter of the wire may be reduced to approximately twentymicrons (20 μm). The strength of the bond between the smaller diameterof the twenty micron (20 μm) wire and the fifty micron (50 μm) wire bondpad opening will be weaker than the bond for a larger diameter wirebecause of the reduced area of contact of the twenty micron (20 μm)wire.

By employing the principles of the present invention, it is possible toreduce the diameter of the wire bond pad opening to approximately thirtymicrons (30 μm) and still use a wire that has a twenty micron (20 μm)diameter. The increased bond strength that is provided by the structureof the present invention wire makes it possible to use wires that have adiameter that is only slightly smaller than the diameter of the wirebond pad opening. The diameter of the wire of the present invention maybe smaller than the diameter of the wire bond pad opening by fivepercent (5%) to twenty percent (20%).

The wires and wire bond pads have been described above as havingcylindrical symmetry (i.e., having a circular cross-section). However,in alternative embodiments, the wires and wire bond pads may beconstructed with other cross-sectional shapes (oval, square,rectangular, irregular, etc.) if required for a particular application.

Although the present invention has been described in detail, thoseskilled in the art will understand that various changes, substitutions,and alterations herein may be made without departing from the spirit andscope of the invention it its broadest form.

1. A structure for receiving a wire bond, said structure comprising: asubstrate material having portions that form a substrate cavity withinsaid substrate material; a wire bond pad covering said substrate cavityand a portion of said substrate outside of said substrate cavity,wherein portions of said wire bond pad fill said substrate cavity toform a wire bond cavity in said wire bond pad; and passivation materialcovering edge portions of said wire bond pad, wherein said edge portionsof said wire bond pad comprise at least part of said wire bond pad thatcovers said portion of said substrate outside of said substrate cavity.2. The structure as set forth in claim 1 wherein said wire bond cavitycomprises portions that form at least one side of said wire bond cavity.3. The structure as set forth in claim 1 further comprising a ball on anend of a wire, wherein said ball is wirebonded to said wire bond cavity.4. The structure as set forth in claim 3 wherein a diameter of said wireis smaller than a diameter of said wire bond cavity by five percent totwenty percent.
 5. The structure of claim 3, wherein the ball iswirebonded to all of a bottom of the wire bond cavity and to all of eachside of the wire bond cavity.
 6. An integrated circuit that comprises atleast one structure for receiving a wire bond as claimed in claim
 1. 7.A structure for receiving a wire bond, said structure comprising: asubstrate material having portions that form a substrate cavity withinsaid substrate material and that form a restraining edge of substratematerial around said substrate cavity; a wire bond pad covering saidsubstrate cavity wherein portions of said wire bond pad fill saidsubstrate cavity to form a wire bond cavity in said wire bond pad,wherein said wire bond pad has portions that form a restraining edgearound said wire bond cavity; and passivation material covering edgeportions of said wire bond pad.
 8. The structure as set forth in claim 7wherein said wire bond cavity comprises portions that form at least oneside of said wire bond cavity.
 9. The structure as set forth in claim 7further comprising a ball on an end of a wire, wherein said ball iswirebonded to said wire bond cavity, and wherein portions of said ballthat fill said wire bond cavity under said restraining edge around saidwire bond cavity form a restraining wedge.
 10. The structure of claim 9,wherein the ball is wirebonded to all of a bottom of the wire bondcavity and to all of each side of the wire bond cavity.
 11. Thestructure as set forth in claim 7 wherein a diameter of said wire issmaller than a diameter of said wire bond cavity by five percent totwenty percent.
 12. An integrated circuit that comprises at least onestructure for receiving a wire bond as claimed in claim
 7. 13. Astructure comprising: a substrate material having a substrate cavity; awire bond pad covering the substrate cavity and having a wire bondcavity over the substrate cavity; passivation material covering at leastone edge of the wire bond pad; and a ball on an end of a wire, iswirebonded to all of a bottom of the wire bond cavity and to all of eachside of the wire bond cavity.
 14. The structure of claim 13, wherein:the wire has a width of approximately 20 microns; and the wire bondcavity has a width of approximately 30 microns.
 15. The structure ofclaim 13, wherein the wire bond cavity comprises portions that form atleast one side of the wire bond cavity.
 16. The structure of claim 13,wherein the wire bond pad has a restraining edge around the wire bondcavity.
 17. The structure of claim 16, wherein portions of the ball thatfill the wire bond cavity under the restraining edge form a restrainingwedge.
 18. The structure of claim 13, wherein a diameter of the wire issmaller than a diameter of the wire bond cavity by five percent totwenty percent.
 19. The structure of claim 13, wherein: the wire bondpad covers a portion of the substrate outside of the substrate cavity;and the at least one edge of the wire bond pad lies outside of thesubstrate cavity.
 20. An integrated circuit that comprises at least onestructure for receiving a wire bond as claimed in claim 13.